Form II of adipic acid–nicotinohydrazide (1/2)

The crystal structure of the title co-crystal, 2C6H7N3O·C6H10O4, is a second polymorph, designated form II, of the co-crystal formed between the two molecules [Lemmerer et al. (2011 ▶). CrystEngComm, 13, 55–59]. The asymmetric unit comprises one molecule of nicotinic acid hydrazide, and one half-molecule of adipic acid (the entire molecule is completed by the application of a centre of inversion). In the crystal, molecules assemble into a three-dimensional network of hydrogen bonds, formed by three N—H⋯O hydrogen bonds and one O—H⋯N hydrogen bond. The O—H⋯N hydrogen bond formed between the carboxyl group and the pyridine ring is supported by a C—H⋯O hydrogen bond.


Comment
Form I of the title compound was obtained by solution crystallization, where 1 equivalent of adipic acid was dissolved with two equivalents of niazid in methanol, and then the solution left to slowly evaporate over a few days (Lemmerer et al., 2011). The ratio of the two starting materials was determined by the two expected primary hydrogen bond interactions, namely the two carboxylic acids hydrogen bonding to the single pyridine of two niazid molecules. Form II was obtained by first grinding the two starting materials in the same stoichiometric ratio as form I for 20 minutes, followed by conventional solution crystallization by dissolving the ground powder in methanol. Crystals were obtained after a few days from the methanol solution. Obtaining polymorphs by either grinding or solution crystallization is a focus of recent research Skovsgaard & Bond, 2009;Karki et al., 2009).
The crystallographic asymmetric unit of the resulting crystal structure of form II reflects this stoichiometry, containing one niazid molecule in a general position and one half adipic acid molecule on a special position (Fig. 1). The two molecules lie approximately co-planar. The expected heterosynthon is formed between two niazid and one adipic acid molecule lying on a crystallographic center of symmetry (which requires the pyridine ring and carboxylic acid to be co-planar). The heterosynthon between the dicarboxylic acid molecule and the pyridine ring is formed by a O-H···N hydrogen bond, as well as a C-H···O hydrogen bond, to form a R 2 2 (7) ring (Bernstein et al., 1995). The niazid molecules are connected by a centrosymmetric R 2 2 (10) ring using N-H···O hydrogen bonds from one of the amine H atoms. Adjacent dimers are joined by a N-H···O bond using the second amine H atom, which together with the C-H···O hydrogen bond and the N-H···O hydrogen bond from the amide H forms a R 2 4 (13) ring (Fig. 2). Hence, all four H bond donors are used to form a 3-D network. In form I, the same R 2 2 (7) ring is observed but the amide H atom forms a C(4) chain to form a 2-D sheet, which is then connected into a 3-D network by the amine H atoms hydrogen bonding to adjacent sheets (Lemmerer et al., 2011).

Experimental
A stoichiometric amount in the ratio of 2:1 of nicotinic acid hydrazide to adipic acid was ground together in a mortar with a pestle under the drop-wise addition of methanol over 20 minutes. The resulting powder was then dissolved in 10 ml of AR-grade methanol, and the solution slowly left to evaporate to afford colourless block-like crystals after one week.

Refinement
The C-bound H atoms were geometrically placed (C-H bond lengths of 0.95 (aromatic CH) and 0.99 (methylene CH 2 ) Å) and refined as riding with U iso (H) = 1.2U eq (C). The N-bound and O-bound H atoms were located in the difference map and coordinates refined freely together with their isotropic displacement parameters.  Fig. 1. The asymmetric unit of (I) extended to show the entire dicarboxylic acid and showing the atomic numbering scheme. Displacement ellipsoids are shown at the 50% probability level. Symmetry code: (i): -x, -y, -z.

Special details
Experimental. Absorption corrections were made using the program Crysalis Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm in CrysAlisPro.